Lacing of compound steel columns is a technique used to enhance the stability and load-carrying capacity of columns in steel structures. It involves the use of diagonal braces or plates that are connected to the flanges and webs of the columns. The main purpose of lacing is to provide additional support to the column against buckling and bending under external loads.

The correct answer is option B, which states that lacing assures unified behavior. This means that the use of lacing ensures that the column behaves as a single unit rather than individual components. The following are the reasons why lacing assures unified behavior:

1. Reduces the Risk of Local Buckling: Lacing provides additional support to the column, which reduces the risk of local buckling. Local buckling occurs when a section of the column is subjected to high compressive stress, causing it to deform or buckle. Lacing helps to distribute the stress evenly across the column, thereby preventing local buckling.

2. Increases the Load-carrying Capacity: Lacing increases the load-carrying capacity of the column by providing additional support against external loads. The diagonal braces or plates used in lacing are designed to resist tensile and compressive stresses, which enhances the strength and stiffness of the column.

3. Enhances the Stability of the Column: Lacing enhances the stability of the column by reducing the overall buckling of the column. Buckling occurs when a column becomes unstable and fails under compressive stress. Lacing helps to distribute the stress evenly across the column, thereby enhancing its stability.

In conclusion, lacing is an effective technique used to enhance the stability and load-carrying capacity of compound steel columns. It assures unified behavior by reducing the risk of local buckling, increasing the load-carrying capacity, and enhancing the stability of the column.